Rubber, Polyisoprenes, and Allied Compounds. VI. The Mechanism of Halogen-Substitution Reactions, and the Additive Halogenation of Rubber and of Dihydromyrcene
The chlorination of cyclohexene by chlorine or by sulfuryl chloride (in the absence of a peroxide catalyst) yields substituted as well as additive chlorination products, and the former retain in full the original olefinic unsaturation, as indicated by iodine-value determination ; but whereas sulfuryl chloride forms only the additive dichloride and a monochloroölefin which is substituted exclusively in the 3-position, chlorine yields a mixture of saturated trichloro derivative (chloro-substituted addition product) and isomeric monochloroolefins (3- and 4-substituted), with some additive dichloride. No evidence of substitution at the ethylenic methine carbon atom has been obtained in the chlorination of cyclohexene, dihydromyrcene, and rubber by chlorine. The diminished unsaturation of the chloro-substitution products of the last two hydrocarbons is attributed to cyclization—a process which is complete in the case of rubber, but affects only a minor proportion of the molecules of dihydromyrcene. The same cyclizing tendency appears in the substitutive bromination of rubber by N-bromosuccinimide, but not to any appreciable extent in the similar bromination of dihydromyrcene. Additive chlorination products are formed when rubber is brought into reaction with chlorine liberated by the thermal dissociation of phenyliododichloride, or of sulfuryl chloride in the presence of a peroxide. The mode of reaction of bromine with rubber, about which many contradictory statements have been made, is found to be entirely additive if the solvent contains a trace of alcohol and the temperature is 0°. A method based on bromine addition may be used for estimating rubber hydrocarbon. Additive bromo and chloroderivatives of rubber are comparatively stable, and give no indication of the spontaneous elimination of halogen acid, either through cyclization reactions or the reformation of double bonds at temperatures up to 80°. The provision of chlorine in free-radical form appears to be an essential condition for obtaining the wholly additive chlorination of rubber and allied olefins. The reaction of molecular chlorine or bromine, on the other hand, follows a course which can be adequately explained by the initial formation of an activated dihalide, the fate of which is determined by the nature of the olefinic system and the experimental conditions.